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Copyright ^ 0 _ 

COPYRIGHT DEPOSIT. 













THIS PHYSICAL WORLD 


UNIFORM WITH THIS VOLUME 


HOW THE WORLD BEGAN 

The Story of the Beginning of Life on Earth 

HOW THE WORLD GREW UP 

The Story of Man 

HOW THE WORLD IS RULED 

The Story of Government 

THE WORLD OF ANIMALS 

The Story of Animals 

THE GARDEN OF THE WORLD 

The Story of Botany 

HOW THE WORLD IS CHANGING 

The Story of Geology 

THE WORLD’S MOODS 

The Story of the Weather 

WHAT MAKES UP THE WORLD 

The Story of Chemistry 

OTHER WORLDS THAN THIS 

The Story of Astronomy 


Thomas S. Rockwell Company 
Publishers 
CHICAGO 










Publishers' Note 


This book presents in popular form the 
present state of science. It has been reviewed 
by a specialist in this field of knowledge. An 
excerpt from his review follows: 

'7 am glad to see this successful 
effort to popularize Physics and 
bring some of its simpler laws within 
the grasp of younger minds." 


Signed Albert A. Michelson 
Professor Emeritus of Physics 
The University of Chicago 







The gravity of the moon attracts the water in the 
ocean which tries to follow the moon 











THIS PHYSICAL WORLD 


‘'Ey 

Janet Pollak 

9 n 

Drawings by 
Lenard Holmes 



THOMAS S. ROCKWELL COMPANY 

CHICAGO 

1930 



Copyright, 1930, by 

THOMAS S. ROCKWELL COMPANY 

Chicago 


Printed in the United States of America 


/ 



lr nri 


©CIA 28713 


CONTENTS 


I Gravity ii 

Why do things jail down? What maizes 
tides? Why is the earth round? Why can 
we drin\ through a straw? How does in\ 
get into a fountain pen? Why can we not 
feel air pressing against us? Why do we 
say water see\s its own level? Why is it 
easier to swim in salty water? What \eeps 
balloons up in the air? Why is it hard to 
balance on one foot? How does a submarine 
stay under water? 

II Push and Pull 34 

Why can we not put Humpty Dumpty to¬ 
gether again? Why do our clothes get wet 
when it rains? Why is it harder to erase in\ 
mar\s than pencil mar\s? Why do we slip 
when we wal\ on ice? Why do we slip in 
the bathtub? Why do we get fire by rubbing 
two sticks together? Why do automobile 
tires wear out? Why do we oil machinery? 

III What Heat Can Do 48 

Why does water sometimes turn to ice? 
What ma\es water boil? Why do doors 
stic\ in the summer time? Why do floors 
sometimes ma\e a crackling sound? Why 
can cement roads not be made in one piece? 

IV Things in Motion 57 

Why does a ball bounce? How can a top 
stand on its point when spinning? What 
\eeps the earth in its orbit? Why does a 
bicycle \eep moving after we stop pedaling 


it? Why is it hard to stop roller spates 
quickly? Why does swimming ma\e us 
travel in water? 


V Why We See 


67 


Why is the night dar\? Why can we see 
through a window pane? What is a mirror? 
Why can we see ourselves in a window at 
night? Why do the stars twin\le? What is 
color? What are dyes? What ma\es a 
rainbow? What is iridescent glass? What 
ma\es the s\y blue? Why are we cooler in 
white clothes in the summer time? How 
does a thermos bottle wor\? How is the 
earth warmed? How does a fire warm us? 


VI What We Hear 


88 


What is sound? What ma\es a room sound 
proof? What ma\es an echo? Why do we 
see lightning before we hear thunder? Why 
does a doctor use a stethoscope? What 
ma\es the music in a phonograph? 


VII Electricity 


99 


Why does a compass point north? How is a 
compass needle magnetized? Why do we 
get a shoc\ if we touch someone after walk¬ 
ing on a carpet? What is an electric cur¬ 
rent? What is a conductor of electricity? 
What is insulation? Why can birds sit on 
electric wires without being hurt? What 
happens when a fuse blows out? What 
causes the light in an incandescent lamp? 
How do we get heat by electricity? 


LIST OF ILLUSTRATIONS 


THE MOON ATTRACTS THE WATER frontispiece 


THE SODA WATER RUSHES UP THE STRAW 17 

WATER WILL SEEK ITS LEVEL 21 

WHEN IRON WILL FLOAT 25 

THE LEANING TOWER OF PISA 31 

WE SLIDE ON THE WATER 41 

ICE NEEDS MOPE ROOM AS IT FREEZES 51 

THE MOLECULES NEAR THE TOP JUMP AWAY 53 
WHY THE BALL BOUNCES 58 

ALL THE MOLECULES ARE MOVING AROUND 61 

LIGHT RAYS TRAVELING TO THE MIRROR 72 

A PRISM WILL BEND A RAY OF SUNLIGHT 77 

THE RAINBOW IS DIVIDED SUNLIGHT 79 

THE THERMOS BOTTLE HAS A DOUBLE WALL 83 
LIGHT TRAVELS MUCH FASTER THAN SOUND 95 
THE AIR CARRIES THE SOUND 96 

BIRDS CAN SIT ON ELECTRIC WIRES 107 




Chapter I 


GRAVITY 

ALL around us every day things happen so 
J- ^-naturally that we never stop to think about 
them and wonder why they always happen just 
that way. But once in a while we do stop in 
our work or play and try to figure them out. 

There is one fact that affects everything we 
do and see. That is the fact that things always 
fall down. If a ball is thrown up in the air, it 
never stays there; it comes right down again. 
When the stem of a leaf is loose from the branch, 
it never floats way up into space and stays there; 
the leaf flutters down to the ground. It is con¬ 
venient for us that this is so and we feel it is 
right that the world should be this way. But 
when we stop to think, we sometimes wonder 
how it happens that things always fall down 
and never up. 


Why do things 
jail down? 


1! 


12 


THIS PHYSICAL WORLD 


If we ask someone about it, we are told that 
gravity makes things fall down. Then we 
want to know what gravity is. Gravity is a 
force which attracts one thing to another. This 
force called gravity works something like a mag¬ 
net. Any two things in the universe try to draw 
close to each other. The bigger a thing is, the 
more power it has to pull other things toward 
it. The earth is so huge—so much larger than 
anything upon it—that it has more of this 
power than anything on the earth. That is why 
it can pull everything down to it. 

When someone throws a ball up in the air, 
this great force in the earth will pull the ball 
down again. The earth is really moving up 
towards the ball, too. But the earth is so very 
big that it moves up only the tiniest bit. It 
moves so very little that we cannot notice it 
move at all. All we really can see is that the 
ball moves down to the earth. A leaf falls down 
because the earth is always pulling on it, and 
when the stem gets weak enough it breaks off, 
and the leaf is pulled down to the ground. 


GRAVITY 


*3 


If we throw a stone out of the window it 
falls because the earth is pulling it. The longer 
the pull lasts the faster the stone will fall, 
although the earth’s pull (the weight of the 
body) does not change. 

The stone will gain more speed when dropped 
from the top of a high building than when 
dropped from a first story window, because the 
pull of gravity has had a longer time in which 
to increase its speed. 

A body moving at a given speed is said to 
have a certain amount of momentum, and any 
force, a push or a pull, must act for a certain 
length of time to generate a given amount of 
momentum. The greater the force, and the 
longer the time, the more the momentum 
will be. 

An automobile or a railroad train always 
starts slowly and gains speed steadily as the 
engine pulls. 

The moon is much nearer the earth than any 
of the stars, and it has this same power of 
gravity that the earth has. Because it is quite 


What makes 
tides and 
waves in 
the ocean? 


!4 


THIS PHYSICAL WORLD 


Why is the 
earth round? 


near the earth, the moon attracts the water in 
the ocean. The moon is not close enough to 
make the water rise up toward it very high, 
but it does draw the water up a little. On 
the side of the earth nearest the moon the 
water of the ocean is pulled by the force of 
gravity more, and on the side away from the 
moon the water is pulled less, than the solid 
earth between. The change in the water level 
is not much but it is enought to cause high tides 
on the sides of the earth nearest to and farthest 
from the moon. Low tides occur at the points 
midway between the high tides. As the moon 
moves around the earth the tides follow it. At 
any place there are two high and two low tides 
each day. 

All other bodies of water, such as lakes and 
seas, have tides, too. But they are so much 
smaller than oceans, that they are not pulled 
very much by the moon, and we do not notice 
that the water shifts from one side to the other. 

Before we can understand why the earth is in 
the shape of a ball, or sphere, we have to know 


GRAVITY 


i 5 


something about what it is made of. Every¬ 
thing in the whole universe is made up of tiny 
particles called molecules. The earth, the stars, 
the air, and everything we know about is made 
up of millions and millions of molecules. They 
are so small that it is impossible to see one even 
under a powerful microscope. 

They are so small that it would be impossible 
to count the number of molecules there are in 
the piece of metal that is used for the head of a 
pin. The reason there are different kinds of 
substances in the world is that there are different 
kinds of molecules. Some of them go together 
to make wood, some make iron, some make 
water, some make air, and so on. Molecules 
always try to get as close together as possible, 
although they are always moving about. 

The earth was made from a part of the sun 
which flew away from the sun into space. 
When that happened, the part of the sun that 
was to be the earth was nothing but a mass of 
burning liquid and gas. Gradually it cooled 
off and changed to what we know as the earth 


Why can 
we drin\ 
soda through 
a straw? 


16 THIS PHYSICAL WORLD 

today. But while it was still hot, the molecules 
were spinning and jumping around very fast, 
and trying to crowd just as close together as 
they could. And as they crowded together and 
tried to fit into as small a space as possible, they 
formed into a great ball. They took that shape, 
because a sphere takes up less room for the same 
amount of matter than any other shape does. 

If we put one end of a straw into a glass of 
soda water, and the other end into our mouth, 
we can draw the soda up through the straw very 
easily. Usually we do this without having to 
think about what we are doing. But if we 
know that gravity pulls everything down, we 
wonder how it happens that the soda water can 
rise up in the straw. If we stop to think about 
what we are doing, we notice that we draw in 
our lips as if we were sucking up air from the 
straw. That is exactly what happens. As we 
suck in the air, the soda water rushes up into 
the straw to take the place of the air. 

As soon as we take the end of the straw out 
of our mouth, the air around it will push its 



The soda water rushes up the straw to ta\e the 
place of the air that was suc\ed out 


17 
























18 THIS PHYSICAL WORLD 

way back into the straw, and force the soda 
water out of the straw and into the glass again. 
This happens because air has pressure. Al¬ 
though we cannot see it, air is always pressing 
against everything. We cannot feel it press¬ 
ing against ourselves because it is pushing as 
much from one side as from the other. There 
is never a space in the world that is really empty. 
If nothing else is there, it is filled with air. 
When we first put the straw in our mouth it is 
filled with air which keeps out the soda water. 
But as soon as we draw in the air, something 
else must take its place, because the air over the 
soda water is pressing down on it. There is no 
air left in the straw to press back. So the soda 
water is pushed up into the straw. 

If we take the straw out of the glass, and close 
one end of it, and then draw in the air from the 
other end, the straw will flatten out. We can 
then see that air has pressure. When there is 
air inside the straw it pushes just as hard as the 
air is pushing outside the straw. But if we take 
away the air inside, the pressure of die air out- 


GRAVITY 


19 


side the straw will push the sides of the straw 
together. Instead of being round and hollow 
like a tube, the straw will look like a flat strip. 

Before we put ink into a new fountain pen, 
the tube inside the pen is filled with air. If we 
set the pen in a bottle of ink, nothing happens, 
because this air presses on the ink and keeps it 
from rising up in the pen. But when we push 
the button at the top of the pen, we force the air 
out of the rubber tube and out of the pen. When 
we release the button there is a vacuum in the 
rubber tube and the ink rushes up to take the 
place of the air. This happens because the air 
around the ink presses down on it and pushes 
it into the empty tube. As long as there is air 
in the tube, it pushes just as hard on the ink 
as the air around the ink does, and the ink will 
stay in the bottle. 

To open a new can of evaporated milk, we 
punch two holes in the top of the can. We do 
this because if we punch only one hole in it, the 
milk does not come out easily. A little milk 
will drip out that way, but it takes a long time 


How does the 
in\ get into 
a fountain pen? 


Why do we 
ma\e two holes 
in a can 
to get a 
liquid out? 


20 


THIS PHYSICAL WORLD 


Why can 
we not feel 
air pressing 
against us? 


Why do we 
say water 
see\s its 
own level? 


to get enough to use. The reason the milk 
doesn’t pour out of a single hole is that the air 
outside is pressing on the milk, trying to force 
it back into the can. But as soon as we make 
another hole, the milk will run out freely. It 
can do this because the air pushes against the 
milk at one of the holes, and as it forces its way 
into the can, it drives the milk out of the other 
hole in the can. 

Although we cannot see air, it has a great deal 
of pressure. It is pushing against us all the 
time. And if we didn’t have something inside 
us to press just as hard as the air presses from 
outside, we would be crushed by this outside 
pressure. But the air inside our bodies, and the 
blood press just as much as the outside air does, 
so that we do not notice the force of the air 
against our bodies. 

When we turn on a faucet, and water runs 
out, it does this because gravity pulls the water 
down. But if gravity pulls everything down¬ 
ward, how does water rise in pipes to all the 
different stories of a house, no matter how high 


GRAVITY 


21 


the building is? We are told this happens 
because water seeks its own level. That means 
that water will always flatten out on its surface. 
If we scoop up a bucketful of water from a well, 



The water will run in the pipes of the building fust 
as high as it stands in the tower 


it doesn’t leave a hole in the water. The water 
will immediately flatten out and fill up the 
space. To do that, the water pushes down. 

It is easy to see how water seeks its own level, 
if we trace it back from the faucet to the place 
where the water supply is kept. On some city 
houses, we see great tanks full of water on the 
roofs. Sometimes all the water for a city is 
kept in a big reservoir which is built in a place 













22 


THIS PHYSICAL WORLD 


that is just as high as the highest building. The 
water is kept there because water will always 
rise up in the pipes to the same level as the water 
in the reservoir. Whenever any water is used, 
the water that is left in the reservoir will flatten 
out and this pushes the water in the pipes until 
it is just as high as the water in the reservoir. 
As long as the water supply is kept just as high 
as the tallest building, the water will run up 
through the pipes to the top floors of the build¬ 
ings. Then if we turn on the tap, the water 
will rush out and gravity will make it flow 
downward, into the drain. 

The water that spurts up in a fountain will 
rise almost as high as the water in the reservoir 
it comes from. In the country, water usually 
comes through pipes from a well near the house. 
If the well is on a hill so that it is just as high as 
the top of the house, the water will rise in the 
pipes to all the floors of the building. But if the 
top of the house is higher than the well, the 
water will not rise up all the way. That is why 
in some farmhouses we can have running water 


GRAVITY 


23 


only in the kitchen and do not have any lava¬ 
tories upstairs where the bedrooms are. 

If we shake up a bottle of milk, the cream 
mixes with the plain milk. But if we let it 
stand quietly for a while, all the cream will rise 
up to the top of the bottle again. What is 
really happening is that the plain milk is sink¬ 
ing to the bottom of the bottle because it is 
heavier than cream. Liquids that are heavy 
will always sink and force lighter substances 
toward the top. 

We know that iron is heavier than water, 
because if we drop a piece of iron in water, it 
sinks to the bottom right away. Wood is 
lighter than water, and we often see pieces of 
wood floating in the ocean. So for hundreds 
of years men made all their ships of wood be¬ 
cause they knew it would float. They were 
afraid to build iron ships, even though iron is 
stronger and would last longer. They knew 
that iron sinks and did not know how to build 
a ship of iron so that it could float. But at last 
someone found a way to build a ship of iron or 


Why does 
cream always 
rise to the 
top of the 
bottle? 


Why do iron 
ships float? 


24 


THIS PHYSICAL WORLD 


steel that would float. To understand how 
this was done, we have to know why some 
things float and others do not. We say a thing 
can float in water because it is lighter than the 
water. What we mean is that the thing is 
lighter than the water that it displaces. When 
we drop a piece o£ wood into the ocean, the 
wood pushes enough water away to make room 
for itself. The wood is equal in weight to the 
amount of water it pushes away, so it floats. But 
when a piece of iron pushes enough water away 
to make room for itself, it weighs more than 
that amount of water, so it sinks. So if a boat 
were made of solid iron, it would weigh more 
than the water it would push away, and the ship 
would sink. But iron or steel ships are made 
with great empty spaces which are filled with 
air* Air is much lighter than water. So the 
iron ship, with all the air that is in it, does not 
weigh as much as the water that it pushes away 
when it slides into the ocean. That is why it 
is able to float. 

Anyone who has tried swimming in the 



When the iron pushes away a greater weight of 
water than the weight of the iron, it will float 


25 























































































26 


THIS PHYSICAL WORLD 


Why is it 
easier to 
swim in 
salty water? 


ocean, and in a lake or brook, knows that it is 
easier to keep afloat in the salty water of the 
ocean than in the fresh water of a lake or brook. 

We weigh the same amount in either kind of 
water. We also do the same things to keep 
ourselves on top of the water, but we don’t have 
to work as hard to stay there in the ocean. The 
reason is that when we push away a certain 
amount of ocean water to make room for our¬ 
selves, we are really pushing away water and 
salt. This water and salt together weigh more 
than pure water does in the lake. So we say 
that salt water weighs more than fresh water. 
Because it weighs more, it is easier for us to 
stay afloat in salty water than in fresh water. 

But our bodies are heavier than any kind of 
water, and if we did not breathe while swim¬ 
ming we would sink. Every time we take a 
breath of air into the lungs, they expand to make 
room for the new air. As the lungs spread out, 
they make us a little bit bigger. Then we take 
up more space in the water, and have to push 
more water out of the way. But because the air 


GRAVITY 


2 7 


that fills the lungs is much lighter than the 
water it keeps us floating, just as iron ships float 
because their extra spaces are filled with air. 

We know that icebergs are nothing but water Why do 
which has frozen. Now because the surface tceber S s fl oat? 
of the ocean is flat, except for the waves, we 
would expect frozen water to have the same 
weight as when it is not frozen, and to fit into 
the same space. If that were true, icebergs 
would not stick up out of the water. But water 
has a strange habit. Instead of becoming 
larger or expanding when it is warm, and 
shrinking or contracting when it freezes, it gets 
bigger when it turns to ice. So the iceberg has 
to push some more water out of the way in 
order to have enough room for itself. The ice¬ 
berg weighs exactly as much as all the water 
that is pushed aside, and so it floats on top of the 
water. It is fortunate that this happens. For 
if icebergs did not float, we could never see 
them, and ships would not be able to steer out 
of their way. 

The Graf Zeppelin traveled all around the 


28 


THIS PHYSICAL WORLD 


What \eeps 
balloons up 
in the air? 


Why is it 
hard to 
balance on 
one foot? 


world in the air. It carried many passengers, 
and food and places to sleep. It has machinery, 
which is very heavy. The big silk balloon part 
is covered with a metal called aluminum. 
Every one of these things is heavier than the air. 
If a man had stepped out of the airship, he 
would have fallen right down to earth. The 
machinery would not stay up by itself. Yet the 
Zeppelin with all these things aboard it could 
float high up over the earth. This was made 
possible, because the silk balloon inside the 
aluminum was filled with hydrogen. Hydro¬ 
gen is a gas which is much lighter than air. 
Things float in air, the same way they do in 
water. There was so much hydrogen in the 
airship that even with all the other things in 
it, the Zeppelin did not weigh any more than 
the air it pushed away to make room for itself. 
That is why it could float up in the air. 

Whenever we stand on one foot, we find it 
difficult to keep our balance. We sway back 
and forth, in trying not to fall. And when we 
see that we are about to fall, we can save our- 


GRAVITY 


29 


selves by quickly putting down the other foot. 
As soon as both feet are on the ground again, it 
is easy to keep our balance. 

Our two feet are the base on which our body 
is built, just as the foundation of a house is the 
base on which it is built. When we stand on 
both feet, the center of our weight is directly 
over them. Nothing can stand erect unless 
the center of its weight is directly over its base. 
When we stand on only one foot, that makes 
the base very much smaller. Then the center 
of weight in our body is not directly over the 
foot. And when the center of weight of any¬ 
thing is not directly over the base, it cannot 
stand upright but will always fall. 

A building can stand, no matter how many 
stories high it is, if the center of its weight is 
over the base of the building. But if a straight 
line drawn from the center of weight in the 
building, down to the ground, touches the 
ground outside the base of the building, the 
building will fall. 

Many hundreds of years ago, in a town in 


30 


THIS PHYSICAL WORLD 


Why does the 
Leaning Tower 
of Pisa not 
topple over? 


How does a 
submarine stay 
under water? 


Italy, called Pisa, a beautiful tower was started. 
When the tower was about thirty-five feet high, 
the builders found that it had begun to sink 
into the earth on one side. In those days it 
took a great deal of time to build, and so the 
men who were erecting the tower did not want 
to undo their work and begin all over again to 
make the tower straight. They continued to 
build slowly on the slanting foundation, because 
they knew that if the center of weight of the 
whole tower was above the base, the tower 
would stand. They finished the tower in a 
slanting position, and it has remained that way 
for 800 years. 

If submarines were built to travel below the 
surface of the ocean without ever traveling on 
the water as other boats do, it would be easy to 
understand how they were made. In that case 
it would be necessary only to make them heavier 
than the amount of water they are to replace. 
But the truth cannot be as simple as that, be¬ 
cause if they were built in that way, they would 
immediately sink to the bottom of the ocean. 


r*' 



As long as the center of weight of the tower is over 
the base the tower will stand 


31 










































3* 


THIS PHYSICAL WORLD 


Submarines are made so that they can travel 
on the surface as other ships do. They are also 
able to regulate the depth under the surface to 
which they go. They are built light enough 
to float on the water. But they have in them 
empty chambers filled with air. When they 
wish to dive below the surface, these chambers 
are opened to let in water. This water gives the 
ship added weight, which makes the submarine 
heavy enough to sink. The more water is let 
into these chambers, the farther down the boat 
will sink. 

Submarines also have diving rudders, or 
hydroplanes as they are sometimes called. 
There are two sets of these hydroplanes, a pair 
placed fore and a pair placed aft on the ship. 
Those which are aft, or at the stern of the 
submarine, are like fins on a fish. They help 
stabilize the ship and keep it in a steady, hori¬ 
zontal position in the water when the sub¬ 
marine is traveling below the surface of the 
ocean. The hydroplanes placed forward, or 
near the bow of the submarine, are movable. 


GRAVITY 


33 


They can be tilted at varying degrees to aid 
the ship in diving to a lower depth in the water 
or rising nearer the surface. Just as the wings 
of an airplane are tilted at different slants so 
the airplane can rise higher or descend to a 
lower level in the air, these diving rudders at 
the fore of the submarine make it easier for the 
ship to move up or down in the water. 

There are also in the submarine tanks of com¬ 
pressed air. This compressed air has more 
pressure than ordinary air. When the ship 
wants to rise to the surface, this compressed air 
is pumped into the chambers, and it forces the 
water out again. 

The farther down into the ocean we go, the 
more pressure there is in the water. So sub¬ 
marines cannot sink very far or they would be 
crushed by the pressure of the water. At a 
distance from shore, the ocean is many miles 
deep, but a submarine does not go down much 
below the surface. 


Chapter II 


PUSH AND PULL 


Why can 
we not put 
Humpty Dumpty 
together again? 


H UMPTY DUMPTY was really made of 
an egg, and if we drop an egg on the floor, 
we know that we can never put all the pieces 
together and make it whole again. Why? Just 
as every other kind of matter is, the eggshell is 
made up of millions of tiny particles which are 
called molecules. 

All molecules in an eggshell are of the same 
kind, and they stick together to make the egg¬ 
shell one smooth piece. When molecules do 
this, we say they cohere or have cohesion. 
Without this cohesion nothing in this world 
would be solid. Everything would be a gas 
like the air, because molecules in the air do not 
stick together. The closer they stick together, 
the harder or tougher a thing is. The mole¬ 
cules in the eggshell stick quite close together 


PUSH AND PULL 


35 


and make the shell hard, but it is thin and not 
very tough. 

Because it is thin, it breaks into many pieces 
when we drop it. This happens because it hits 
the floor with a great deal of force and this 
makes the molecules separate in some places so 
that we have a lot of little pieces instead of one 
whole eggshell. Once it has broken, it cannot 
be put together again, because we can’t get the 
molecules near enough each other to make them 
stick. If the pieces are not too small, we might 
make them stay together with glue. We could 
do this because glue is sticky, which means it 
has a great deal of adhesion and can make two 
parts stick together. Part of the glue touches 
one piece and part touches another piece. All 
the particles in the glue will stick together and 
thus hold the broken pieces together. 

There are many things that have great cohe¬ 
sion. We can pull and pull on a piece of iron 
and not separate one part from another. Only 
when we hit it very hard with something strong 
will it break. And after it is broken, it cannot 


THIS PHYSICAL WORLD 


Why do our 
clothes get 
wet when 
it rains? 


36 

be put together again unless it is heated. When 
iron is made very hot, it becomes soft. If the 
two pieces are hammered together while they 
are soft, the iron will form one single piece again 
when it gets cool and hard. This is what is 
called welding. 

If the gravity of the earth makes raindrops 
fall to the ground from the clouds, then all the 
drops that touch us as they fall should roll right 
off again. And if gravity pulled all the rain¬ 
drops off our clothes, we should be perfectly 
dry even if we had no umbrella. But there is 
a power in the water which makes some of it 
stick to our clothes, instead of falling right down 
to earth. The molecules in the water will try 
to stick to the things they touch, in the same 
way they try to stick together. The gravity in 
the earth is so strong that it will pull most of the 
rain down to the ground. But the raindrops 
that touch our clothes will stay there. This is 
because the power to stick is stronger, when 
things are close together, than the gravity of 
the earth. 


PUSH AND PULL 


37 


If we put our hands in a basin of water, some 
of the water will stick to our hands when we 
take them out. Gravity will pull on most of 
the water and keep it down in the basin. But 
the water that is very close to our hands will 
have enough power to stick to the skin, in spite 
of the force of gravity which is trying to pull 
it off. This power which molecules have to 
stick to things is called adhesion. 

If we touch a pool of water and then raise a 
finger a little bit, the water will cling to the 
finger. That is because the water that is close 
to the finger is trying to stick to it. But gravity 
is so strong that most of the water will stay 
down. Only a little of the water that really 
touches the finger will stay on it when we lift 
the finger up higher. 

When we are turning the pages of a book, 
sometimes we wet our fingers and that makes 
our task easier. This is because the water on 
the fingers tries to stick to the page and to the 
fingers at the same time. When we move the 
fingers, the page moves with them. 


38 


THIS PHYSICAL WORLD 


Why is it 
harder to 
erase in\ 
mar\s than 
pencil mar\s? 


When we use an eraser to rub out what we 
have written on a piece of paper with a pencil, 
it is usually easy to get the page quite clean 
again. None of the marks left by the pencil 
are left, and we can write in the same space 
again. But when we have written something 
in ink, we cannot rub out every bit of it. Some 
of the ink-marks remain on the paper, no matter 
how hard we rub. The reason for this is that 
when we make marks with a pencil one thing 
happens to the paper, and when we make marks 
with ink a different thing happens. 

As we move a pencil across a page, tiny par¬ 
ticles of the lead are left wherever the point 
touches the paper. The particles stick to the 
paper because of adhesion. Wherever the point 
touches the paper, the molecules in the lead pull 
off from the whole piece of lead and stay on the 
paper. But these particles stay on the surface 
of the paper. They do not sink into it, or 
become part of the paper. We notice this if we 
brush our hand across the page. The writing 
will get smudged. So when we rub the page 


PUSH AND PULL 


39 


with an eraser, little particles of lead come off 
the paper and stick to the eraser, or get mixed 
with the tiny pieces of rubber that remain like 
crumbs on a page when we have finished 
rubbing with the eraser. 

But when we write in ink, part of the ink 
soaks into the paper, and the part which stays 
wet on top evaporates. When we try to erase 
these marks after they are dry, we cannot get 
them all out without rubbing a hole in the page. 
The ink has gone right into the paper and 
become part of it. The power which the paper 
has to draw ink into it is called capillary attrac¬ 
tion. If we could write with ink on a piece of 
glass, it would be quite easy to rub it all out 
again when it was dry. This is because glass 
does not have the power to absorb a liquid. 
Things that can absorb a liquid have tiny empty 
spaces in them between the particles of which 
they are made. Usually these spaces are so 
small that we cannot see them. But they are 
like very small tubes, and the liquid will run 
up into these tubes. We cannot see the spaces 


4 o 


THIS PHYSICAL WORLD 


Why do we 
slip when we 
wal\ on ice? 


in a piece of paper, but they are there, and the 
ink runs into these tube-like places. The kind 
of paper that blotters are made of has even more 
spaces in it than ordinary writing paper. That 
is why we can use it to absorb large blots of ink 
or paint or water. In glass there are no such 
spaces between the particles. So the ink stays 
on the surface of the glass and can be very 
easily erased. 

If we take two lumps of loaf sugar, and set 
them in a saucer one on top of the other, and 
put just a few drops of coffee in the bottom 
of the saucer we can see easily how capillary 
attraction works. If we watch for a few sec¬ 
onds, we will see first the lower lump of sugar 
become brown in color as the coffee rises up 
into the tube-like places in the sugar. And if 
we watch for a little longer, the coffee will also 
rise up into the second lump of sugar. 

When there is ice on the street, it is much 
harder to walk without slipping and falling 
than when the streets are dry. And it is also 
harder to walk on a bare floor that has been 



The rubbing of the shoe mattes a little of the ice 
melt and we slide on this water 


41 







42 


THIS PHYSICAL WORLD 


polished than on a rug or carpet. To under¬ 
stand why this happens, we have to learn why 
we do not slip on a clean street or on a carpet. 

Each time we put a foot down, the sole of our 
shoe rubs against the asphalt or cement of the 
sidewalk. This rubbing holds the foot down 
until we put the other foot forward, and lift up 
the first foot. The same thing happens when 
we walk on a carpet or rug. When two things 
rub against each other this way, we say they 
cause friction. If we could look at a piece of 
the street under a microscope, we could see that 
it really is not very smooth or even. We could 
see that there are tiny bumps and ridges in it. 
When things are not very smooth they have 
friction. 

Now things that are smooth have almost no 
friction. Ice is fairly smooth but it has some 
friction. Now when our foot comes down on 
the ice, the rubbing of the shoe on the ice makes 
a tiny bit of the surface melt and form drops of 
water, which has practically no friction. Our 
shoes slide on this film of water. So if we are 


PUSH AND PULL 


43 


not careful as we walk on the ice our feet keep 
slipping and we are likely to lose our balance. 

A floor that is waxed becomes very smooth 
and has almost no friction. But a carpet is thick 
and soft, and so it is easy to walk on a carpet 
without slipping. People wax floors when 
they want to dance, because in dancing they 
want their feet to slide along the floor. It is 
difficult to dance on carpet, because the friction 
makes our shoes stick when we try to slide. 
People often sprinkle sawdust or ashes on ice. 
This makes the surface uneven, instead of 
smooth, and then we can walk on it without 
slipping. When our shoes are new, the soles 
are quite smooth. That is why we sometimes 
slip on a clean pavement before the soles of our 
shoes become rough and uneven from wear. 

Most bathtubs are made of porcelain which 
is fairly smooth. But porcelain has enough fric¬ 
tion so that when we stand in a dry tub, we are 
safe and not likely to slip and fall. But if the 
bottom of the tub is wet, there is a film of water 
between the tub and our feet. And as water has 


Why do 
we slip in 
the bathtub? 


44 


THIS PHYSICAL WORLD 


Why do we 
get fire by 
rubbing two 
sticks together? 


What happens 
when a match 
is struc\? 


almost no friction, it is difficult to stand in the 
wet tub without slipping. 

Friction is very important. It gives forth 
heat besides preventing things from slipping. 
We rub our hands together when they are cold, 
because that warms them. It is friction which 
causes that warmth. 

When two hard substances are rubbed to¬ 
gether for a long time they get very hot. If 
we continue to rub, they finally grow so hot that 
sparks fly out. Men first kindled fires by 
rubbing two pieces of wood together until they 
became hot enough to give off sparks. They 
would hold the two pieces over a bunch of twigs 
and leaves, and the sparks would fly into this 
heap. So a fire would be started. 

In a simple way, a match is lighted in much 
the same manner that fires were first built. 
Men struck two stones together because they 
could get a spark more quickly than by rubbing 
two pieces of wood together. They would let 
the spark from the stones set fire to the kindling. 

In the old matches, it was not the wood that 


PUSH AND PULL 


45 


first began to burn. Phosphorus bursts into 
flame at a much lower temperature than wood. 
So the top of the match was made of phos¬ 
phorus. When this was rubbed on a rough 
surface, it ignited without becoming very hot. 
Under the phosphorus head of the match was 
a little sulphur. Sulphur needs a little more 
heat to burst into flame. But the flame of the 
phosphorus was enough to make the sulphur 
catch fire. And when the sulphur began to 
flame it gave off enough heat to make the wood 
of the match begin to burn, too. If the whole 
match were made of wood, no matter how long 
a time we rubbed it on sandpaper, we probably 
could not get it hot enough to flame up. 

Friction also makes things wear out. There 
is always friction when the tires of an auto¬ 
mobile roll along the road. If friction did not 
hold the tires to the road, the car would slip 
and slide on a dry road, as it does when it is 
raining. (We know it is the water between the 
tires and the ground which makes a car skid 
in rainy weather.) 


Why do 
automobile 
tires wear out? 


4 6 


THIS PHYSICAL WORLD 


Why do we 
oil machinery? 


The tires rub a little on the road, and as it 
does so, some of the molecules are pulled off 
from the whole tire so far that they do not 
jump back on again. The tire also wears out 
because in going over uneven and bumpy roads, 
little pieces of the tire are torn off. 

Of course, they are so small that when a few 
of them jump off we cannot notice it. But 
after a long time, so many of them have jumped 
off that quite a bit of the tire has been worn 
away, and it cannot be used any more. 

Our clothes wear out for the same reason. 
Stockings wear out at the knees usually, because 
every time we kneel down to play, there is a 
rubbing between that part of the stocking and 
the ground. Sleeves often wear out at the 
elbow, because every time we bend an arm, the 
cloth is rubbed against the hard, sharp surface 
of our elbow and this causes friction. 

There is always friction between any two 
parts of machinery that touch each other while 
the machinery is in motion. When we put oil 
between the parts, there is much less friction, 


PUSH AND PULL 


47 


because each part touches the film of oil. Oil 
is smooth and so there is little friction when the 
parts of the machine touch oil instead of rubbing 
against each other. By making friction so 
much less, we can make machinery last a great 
deal longer. 

Machinery will also run much more easily 
and quickly when it is oiled. The heat of fric¬ 
tion makes the parts of machinery swell and 
forces them closer together than they are sup¬ 
posed to be. When this happens, it is harder 
for the machinery to work smoothly. By oiling 
the parts, we help to keep them from touching 
each other and so the whole machine will work 
more easily. 


Chapter III 


Why does 
water sometimes 
turn to ice? 


WHAT HEAT CAN DO 
E KNOW that water becomes hard and 



▼V turns into a solid called ice, when the 
temperature goes down to 32 degrees, but this 
happens so naturally and so conveniently for 
us, that usually we do not give any thought to 
it, or wonder why this happens. We also 
know that water turns to steam and disappears 
in the air when it is heated until it boils. There 
are reasons for both these changes in water. 

Water is made up of molecules, as everything 
else is. But in water, the molecules do not stick 
so closely together as they do in solid materials, 
such as wood or steel or glass. 

Heat has the power of dilating: that is, it 
pushes, these molecules somewhat apart from 
each other in spite of their own power to stick 
together. When sufficient heat is applied to 


Why does 
water sometimes 
turn to ice? 


48 


WHAT HEAT CAN DO 


49 


water, it changes to steam. Scientists divide 
things into three groups called gases, liquids and 
solids; such as steam, water, and ice. Steam is 
a gas which contains the same molecules as 
water, but farther apart. In steam, they do 
not stay as close together as they do in water. 

Adding heat to water changes it into steam. 
We are not surprised that an equally interesting 
change occurs when heat is taken away from 
water. As we know, liquid water turns into 
solid ice when enough heat is taken from it. 
If heat makes molecules expand or spread 
apart, cold forces them closer together. If we 
take a pan of water from a warm room and set 
it outside where it is very cold, the molecules 
in the water begin to move closer together. 
When the temperature goes down to the point 
we call freezing (32 degrees Fahrenheit), the 
molecules become so compact that the water 
forms a solid mass called ice. 

Heat causes molecules to expand. Cold 
makes them contract. Usually contraction 
causes shrinkage, or makes a thing smaller. 


5 <> 


THIS PHYSICAL WORLD 


What ma\es 
water boil? 


Most things take up less space when they are 
cold than when they are hot. But water is an 
exception to this rule. Just before water freezes 
into a solid mass, it expands somewhat or be¬ 
comes a little larger. 

If a pan is filled to the brim with water, when 
it freezes the ice will extend out above the top 
of the pan. A bottle filled with water and 
corked tightly will break if the water is frozen. 
The ice needs more room, and as there is none 
in the bottle, it will break the glass as it pushes 
out if it cannot force out the cork. 

In the winter pipes in a building which carry 
water to the faucets will sometimes burst. If 
water is not being used frequently and stands 
still in the pipes, it may freeze in extremely cold 
weather. As ice it will need more room than 
the water did, and will break the pipes as it 
spreads out. 

Water, like everything else, is made up of 
molecules. If we could find a microscope 
powerful enough to show the molecules in water 
or in anything else, we could see that they are 



The ice needs more room as it freezes and it will 
brea\ the bottle as it expands 


51 




















52 


THIS PHYSICAL WORLD 


always moving about. Heat makes these mole¬ 
cules dance around faster. The hotter they get, 
the faster and farther they jump around. When 
a kettle of water is set over a gas flame, the heat 
from the flame comes through the kettle and 
makes the water hot. As the water becomes 
hotter and hotter, the molecules in the water 
jump around faster and faster. Finally, they 
jump so far that the ones near the top of the 
water jump away from the rest right out into 
the air. When the water gets so hot that this 
happens, we say it is boiling. 

When water boils, many of these molecules 
bounce out into the air. If we look carefully 
at a kettle of boiling water, we can notice that 
for a space of about an inch from the open end 
of the spout we see nothing. That is because 
steam is invisible. The molecules that jump 
out are so small we cannot see them. But by 
the time the steam gets about an inch away 
from the spout, the air, which is of much lower 
temperature, cools the steam and it condenses 
into tiny droplets of moisture in the air. This 



Finally the molecules near the top jump away from 
the rest of the water right into the air 


S3 










54 


THIS PHYSICAL WORLD 


Why do tires 
sometimes 
blow out in the 
summertime? 


we call vapour, and it is this cloud of vapour 
which we see rising from the kettle when the 
water is boiling. If we turn the gas flame 
higher after the water has started to boil, we 
cannot make the water any hotter. Instead of 
getting hotter, more and more of the molecules 
will jump out into the air and form steam. If 
we wait long enough there will be no more 
water left in the kettle, because all of it has 
turned to steam in the air. 

When heat makes the molecules in a thing 
try to jump away, they spread out and make the 
thing a little bigger than it was before. 

Sometimes when an automobile is left stand¬ 
ing out in the street with the sun blazing down 
upon it for a long time in the summer, one of 
the tires will burst. That will happen because 
the extreme heat of the summer sun will cause 
the air to expand until it needs more room than 
there is inside the tire. The rubber of the tire 
may expand a little from the heat, but it will 
not expand as much or as quickly as the air 


WHAT HEAT CAN DO 


55 


does. So the air will finally push on the tire 
until it bursts right through. 

In winter we have steam heat in our rooms 
all day to keep them warm. This heat makes 
the wood in the floors swell a little. Then at 
night the heat dies down in the radiators, and 
we open a window to let in fresh air. This 
makes the room cold, and then the wood in the 
floor shrinks back to its right size. As it does 
this, we hear a cracking sound. 

When we drive along a wide road of cement 
or concrete, we notice a strip several inches 
wide which runs along the middle. This strip 
is usually made of tar. And every little while 
we also see a short, twisting strip of tar in the 
road. The strip in the middle was put there 
when the road was made. The road is made 
in two parts with a little empty space in be¬ 
tween. This is done so there will be room for 
the road to spread in the summer when it gets 
hot. After the road has settled, there is still a 
small crack left, and this is filled with tar. 
Sometimes, even after the road is made, it 


Why do floors 
sometimes 
ma\e a 

crackling sound? 


Why can 
cement roads 
not be made 
in one piece? 


56 


THIS PHYSICAL WORLD 


Why do we 
open windows 
both at 
the top 
and bottom? 


shrinks or contracts in cold weather and leaves 
little wriggly cracks. These also have to be 
filled with tar to make the road smooth. If the 
cement were put down all in one piece, it would 
swell when the weather got hot, the road would 
be full of cracks and would wear out quickly. 

To get the best circulation of air in a room, 
we try to let fresh air in and force used air out at 
the same time. Air that we have been breath¬ 
ing becomes warmer than fresh air, because it 
comes from our bodies which are warm. The 
colder the air is, the heavier it is. Heavy air 
will sink down in the room and force the 
warmer air up, toward the top. So we open a 
window a little at the top to let out the used, 
warm air which it up high, and we open a win¬ 
dow a little from the bottom to let in the fresh, 
cooler air. 


Chapter IV 


THINGS IN MOTION 

A BALL made of rubber will bounce. A 
^ ^-hollow ball made of celluloid bounces, 
and so will balls made of some other things, 
such as tennis balls. We know balls do not 
bounce just because they are round, for a ball 
made of string or of dirt or of clay will not 
bounce. Rubber balls bounce because they are 
elastic. 

We say things are elastic when they try to go 
back to their original shape after they have 
been forced to change it. When a rubber ball 
is thrown to the ground, the part that touches 
the earth is flattened out. Because the rubber 
is elastic, it tries to become round again. To 
do this, the ball will push itself away from the 
ground, and that will make it spring into the 
air. Many things are elastic, but some are 
57 


Why does a 
ball bounce? 


5® 


THIS PHYSICAL WORLD 


more elastic than others. Rubber has a great 
deal of elasticity and that is why the ball bounces 
so high. A baseball is made of twine and it 
does not have much elasticity. But it has a 
little bit, and if we bounce a baseball hard, it 



The part that touches the ground flattens out and 
when it becomes round again it pushes itself away 


will jump up a little from the ground. Glass 
is also somewhat elastic, as we can see when we 
make marbles bounce. 

We use rubber bands to bind up packages 
because rubber is so elastic. We stretch the 
rubber band until it goes around the package. 
To do this, it is pulled out of its own shape, and 
so it tries to regain its own size. As it does this, 
it presses very hard on the package all around, 
and so holds it together. 



THINGS IN MOTION 


59 


If we try to make a top stand on its point, it 
falls over as soon as we take our hands away. 
That happens because there is more weight 
toward the upper part of the top, and it natur¬ 
ally falls in a position which brings the center 
of weight toward the ground. But when we 
spin a top, it stands on its point as long as it 
keeps spinning. When it begins to whirl more 
slowly, the top begins to wobble, and at last it 
falls because gravity pulls the heavier part down 
towards the earth. 

We wonder why gravity does not pull the 
heavier part down while the top is spinning, 
as well as when it is at rest. So we are ready 
to learn that there is another power which 
makes things move in a certain direction, be¬ 
sides gravity. This force which causes a thing 
to want to keep on going in the direction in 
which it starts to move is called inertia. 

In whirling things, this force makes bodies 
tend to fly away from the center and is called 
centrifugal force. This is seen plainly when 
one stops to think what happens when a top 


How can a 
top stand on 
its point when 
it is spinning? 


6o 


THIS PHYSICAL WORLD 


What \eeps 
the earth 
in its orbit? 


spins. When a top starts spinning, all the 
molecules in it are moving around and around 
in circles. As they do this, they tend to fly out, 
or away from the center of the top. The faster 
it spins, the harder the molecules try to fly out 
in the direction they first started to go. The 
force which makes them fly away from the 
center is stronger than gravity while the top is 
spinning. The molecules pull just as hard from 
all sides of the top, away from the middle, and 
so the top will balance on its point. But as it 
slows down, the force of gravity is stronger, and 
the top will fall over on its side. 

The sun is so much bigger than the earth 
that its gravity could pull the earth toward it, 
just as the earth pulls on smaller things. But 
the earth is always flying around the sun very 
fast. This makes the whole body of the earth 
try to fly away from the center. The sun is the 
center of the orbit in which the earth travels. 
This force is exactly equal to gravity as long as 
the earth keeps whirling. That is why the 
gravity of the sun does not pull the earth toward 


THINGS IN MOTION 


61 


it. If the earth ever stops whirling around the 
sun, it will crash into that great burning ball 
and be burned up by its heat. 



All the molecules are moving around in circles and 
try to fly out in the direction they started 


If we race a bicycle by pedaling quickly and 
steadily for a while, it continues to go for some 
time after we lift our feet from the pedals. Soon 
it slows down, however, and unless we begin to 
pedal again, it stops moving altogether. 


Why does a 
bicycle \eep 
moving after 
we stop 
pedaling it? 






62 


THIS PHYSICAL WORLD 


It stops because gravity pulls down on the 
wheels, and thus increases friction which tends 
to keep the wheels from going around. But if 
friction tends to keep the wheels from moving, 
why does the bycicle move at all after we have 
stopped pedaling? 

The wheels keep going forward because all 
things continue to move in the direction in 
which they have started. At least, they tend to 
keep moving until gravity or friction brings 
them to a stop. Once the wheels of a bicycle 
have started going forward, they keep going in 
that direction and in a straight line until they 
are stopped by an outside force. If there were 
no friction, the bicycle on level ground would 
continue to go until it ran into something, like 
a fence or a house or a tree. 

It is true that everything tends to keep going, 
once it is started moving, until it is stopped by 
something else. It is also true that a thing 
which is not in motion, will never move unless 
it is started by an outside force. Leaves never 
rustle until the wind pushes them about. Water 


THINGS IN MOTION 63 

in a pond does not move unless the wind ripples 
it. And once our bicycle has stopped, it will not 
go again until we give it power by working the 
pedals. 

No object that is in motion will stop until an 
outside force makes it stop, and no object that is 
standing still will move until it is made to move 
by an outside force. This is called inertia . 

If we are racing along on roller skates and 
wish to stop quickly, we have to take hold of 
something and hang on to it. Even when we 
grasp a post, we have to hang on tightly to keep 
from being pulled away from it, because our 
bodies keep on moving for a while. The skates 
and our bodies try to keep going in the way 
they have started. When we stop by holding 
on to something, the skates go on for a foot 
or two, with our feet on them, and we are 
likely to fall if we don’t hold tight. If we 
are skating fast, and try to stop by taking 
someone’s hand, we often find we cannot come 
to a halt, but instead of this pull the other person 
along with us. This happens because inertia 


Why is it 
hard to stop 
roller spates 
quic\ly? 


6 4 


THIS PHYSICAL WORLD 


also makes our bodies try to keep moving the 
way they were going, even after we strive to 
stop them. 

When we ride on trains, we move forward 
with the train and at the same speed as the 
train. If the train stops suddenly, we lurch 
forward. Inertia makes our bodies try to keep 
going forward just as they were moving before 
the train stopped. 

If it were not for inertia, we could not throw 
things. As soon as we let them go, gravity 
would pull them down just as if we had dropped 
them. But when we want to throw a ball, we 
take the ball in one hand. Then we raise that 
arm, and also draw it back a way. As we bring 
it forward again, we start the ball moving in 
that direction. When we let the ball go, it 
keeps moving, because of inertia. But gravity 
pulls on it, and finally forces the ball to the 
ground. If gravity did not do this, the ball 
would keep moving in a straight line until it 
hit something strong enough to stop it. 

Unless we keep swimming, when we are in 


THINGS IN MOTION 65 

deep water, we find ourselves sinking. Our 
weight does not change because we swim or 
stop swimming, and the weight of the water 
remains the same. So we know that the mo¬ 
tions we make when we swim keep us on the 
surface of the water, as well as make us go 
forward. 

When we are taught to swim, we are warned 
to keep our fingers close together, and not let 
the water get through. In this way our hands 
become flat and solid like a paddle which exerts 
pressure against the water. Every time we bring 
one hand forward overhead, we push it back¬ 
ward through the water. As we do this we 
also push some of the water back. And as we 
push the water back, we force our body forward. 
This is called the law of action and reaction. 
We push the water in one direction: that is 
action. Our body moves in the opposite direc¬ 
tion: that is reaction. While we are in motion 
we keep afloat. 

Action and reaction are always opposite, and 
equal. Without this law, nothing in the world 


Why does 
swimming 
ma\e us 
travel in water? 


65 


THIS PHYSICAL WORLD 


would move. When we take a step forward, 
our foot is really pushing the earth back a little, 
and that makes our foot go forward in the 
opposite direction, with equal force. But the 
earth is so large that it doesn’t move enough 
to be noticeable. The same thing happens when 
the wheel of an automobile moves around. 
It keeps pushing the earth back as it moves 
forward. This can be seen if the car is going 
along a pebbly road. We cannot see the earth 
move back, but we do see the pebbles fly back 
as the car moves forward. 


Chapter V 


WHY WE SEE 

"T^ARKNESS is absence of light. As soon 
as the sun begins to sink, the earth loses 
some of its light. The farther down the sun 
goes, the darker it becomes, and after the sun 
has set and the clouds in the west have no more 
brightness, it becomes so dark that we cannot 
see anything clearly. Therefore, it is easy to 
guess that the earth gets light from the sun. 

Light radiates from the sun in rays, just as 
heat does. One single ray of light cannot be 
seen by itself. But many rays together form a 
beam of light. We often see a beam of sunlight 
shining through a window, or a beam of moon¬ 
light shining down on us at night, reflected 
to us by the particles of dust in the air. 

We see the sun because it is so hot that it 
shines with its own light. Things that shine by 


Why is the 
night dark? 


67 


68 


THIS PHYSICAL WORLD 


their own light are said to be incandescent. We 
see the stars because most of them are burning 
and are incandescent. But people and trees and 
houses and rivers and almost all things that 
we see around us do not have any light of their 
own. We see them because they diffuse light. 

When light rays travel down to us from the 
sun, they touch things they pass. When a light 
ray strikes a thing, it bounces back from that 
object to our eyes. And there is something in 
our eyes which receives the image that the ray 
of light carries to them. If it were not for these 
rays of light which carry the pictures of things 
to us, and if all things did not diffuse light, we 
would never see anything. 

We do not see things as well at night when 
the sun is gone, because there is not enough 
light from the stars to bring us a good image. 
But sometimes when the moon is shining 
brightly we see almost as well as we do by day. 
The moon is never as bright as the sun, because 
it is not incandescent. The moon itself is not 
glowing; it merely reflects light from the sun, 


WHY WE SEE 


69 


and sends it down to us. But it absorbs some 
of the light rays because its surface is not 
smooth. We get more light from the moon 
at night than from the stars, because the moon 
is so much nearer to the earth. If we could 
get up to the moon, we would see the earth 
shining in the sky. It would be reflecting the 
light from the sun to the moon, just as the moon 
reflects the sun’s light to us. 

The sun is many million miles away from the 
earth, yet light travels so fast that we seem to 
see things without having to wait for the light 
to reach us. 

Electric light is also incandescent, and it sends 
out rays of light, just as the sun does, so that 
with its aid we see at night as well as by day. If 
we walk into a room, the rays of light from the 
bulb travel out and when they touch us, they 
bounce back in all directions and will bring an 
image of us to the eyes of other people in the 
room. This happens so quickly that people see 
us as soon as we enter. The time it takes for 
light to travel that short distance is so brief that 


70 


THIS PHYSICAL WORLD 


Why can we 
see through a 
window pane? 


it seems as if they saw us at once. The time is 
only the tiniest fraction of a second. 

Rays of light travel through the air, because 
air is transparent. Things are transparent when 
they allow light rays to pass through, without 
breaking, or diffusing them. Glass is trans¬ 
parent, and that is why it is used for windows, 
so that we can get sunlight into a room. The 
light rays come in unbroken and we are able to 
see the things outside. But some windows are 
made of special glass which will let in the light, 
but through which we cannot see objects on the 
other side. This is usually glass that has been 
frosted. This makes the surface dull and not 
very smooth, so that the light comes through, 
but the rays are broken or diffused, and we do 
not get an image of things on the other side. 
Material which lets in light in this diffused way 
is called translucent. A white window shade is 
translucent. It will let in light, but we cannot 
see through it. But if we pull down a dark 
green or black shade, it will keep out the light. 
What really happens is that as the light strikes 


WHY WE SEE 


7i 


the shade, instead of going right through, it is 
absorbed by the shade. Things through which 
light does not penetrate at all are called opaque. 

Frosted glass is used for electric light blubs 
because all the light comes through so that we 
can see clearly, but the rays are broken and do 
not shine directly into our eyes. This is restful 
and makes artificial lighting less of a strain on 
our eyes. 

A mirror is usually made of glass. Anything 
that has a very smooth, shiny surface reflects 
light well, but if we use plain glass in a mirror, 
most of the light passes right through. Silver 
or mercury is usually put on the back of the 
glass. Silver and mercury are opaque. The 
light rays cannot penetrate them. 

When light rays strike the mirror, they can¬ 
not get through. So they bounce back in the 
opposite direction, and in a straight line. If 
we stand directly in front of the mirror, light 
rays travel straight from us to the mirror. Strik¬ 
ing the mirror, they bounce back again along 
the same line, and we see ourselves. But when 


What is 
a mirror? 


7 2 


THIS PHYSICAL WORLD 


we stand at one side of the mirror, the light must 
travel in a diagonal line from us to the mirror. 
Then when it bounces back from the mirror, it 
will go again in a diagonal line, but in the 



When we stand at one side of the mirror the light 
rays must travel in a diagonal line 


opposite direction, like a ball bounced on a slant 
against a wall. That is why we cannot see 
ourselves when we stand at one side of a mirror. 
But if we stand at one side, and someone else 
stands at the other, we can see him and he can 
see us. 

Anything that is smooth and glossy and 
opaque can be used as a mirror. A flat silver 
dish will reflect things as well as a mirror. If 








WHY WE SEE 


73 


the dish is not flat, the light rays will be bent so 
that we do not get an accurate picture. A pond 
or lake is smooth on the surface, and the water 
is shiny. That is why we see trees and clouds 
reflected in the water. 

Glass window panes are smooth and shiny 
enough to act as mirrors. We find this to be 
true at night, when the room is brightly lighted 
and it is dark outside. Then we see ourselves 
in the window almost as well as we can in a 
real mirror. Everything in the room is reflected 
in the window panes. But in the daytime it is 
difficult to see reflections in a window unless 
the light is just right. 

There is no difference in the glass at night. 
But we see these reflections then because there 
is more light inside than there is outside. The 
reflections are there in the daytime, but light 
coming through the window from the sun is so 
much stronger than the light in the room that 
it drowns out the reflections, just as a loud noise 
will drown out a softer noise. At night there is 
bright light inside the room, and none outside. 


Why can we 
see ourselves 
in a window 
at night? 


74 


THIS PHYSICAL WORLD 


Why do the 
stars twinkle? 


We see the reflections distinctly then because 
there is no more powerful light to hide them. 

In the daytime, we see reflections in windows 
on the outside, for the same reason. If we walk 
along a street and look into a shop-window, we 
often see reflections of ourselves and other 
people walking by. We see them because the 
light outside the window is strong, and there is 
very little light inside to shine out and spoil the 
reflections which we see. 

The stars we see in the sky at night are really 
great round balls like the earth and the moon 
and the sun. They send out light rays just as 
the sun does. But most of them are much 
farther away from the earth than the sun is. 
That is why they look so tiny to us. Some of 
them are burning and give off their own light. 
A few just reflect the light of the sun, as the 
moon does. But all of them are sending light 
rays which travel toward the eatrh. As these 
rays of light travel, they pass through uneven 
layers of air. This unevenness causes the rays 
of light to be bent in different directions as they 


WHY WE SEE 


75 


are traveling down toward the earth. It is 
really only the light rays moving back and 
forth that makes it seem as if the star itself 
were twinkling. 

We say that white things have no color. But What is color? 
white is not the absence of color. It is really all 
colors blended together. Sunlight is white 
light. A beam of light is white. Yet if we 
could see each ray of light in that beam sep¬ 
arately, we would see that it had color. 

There are six different colors in light. The 
longest rays are red. Rays that are a little 
shorter are orange in color. Yellow light rays 
are still shorter. Light rays that are a little 
shorter than the yellow are green in color. Blue 
light rays are shorter than green rays. The 
shortest light rays are violet. 

The eye cannot detect the difference between 
pure orange and a proper mixture of red and 
yellow. A mixture of yellow and blue appears 
green to the eye, and violet may be imitated by 
a mixture of red and blue. Orange, green, and 
violet are sometimes called secondary colors, 


7 6 


THIS PHYSICAL WORLD 


What are dyes? 


because they may be imitated to the eye by 
mixing the socalled primary colors, red, yellow 
and blue. 

These six colors are the only true colors, be¬ 
cause they appear in the light rays. And all 
color that we ever see is reflected from light. 
The many different colors we use are made by 
mixing pigments which will combine two or 
more of these colors. 

When we have something dyed, we know it 
is dipped in boiling water in which some 
powder has been mixed. The powder has in it 
the color we want. At least we say the powder 
or dye is that color. But all color is really in 
rays of light. A certain dye looks red to us 
because that dye absorbs from the light every 
color but red. It reflects the red rays back at 
us so that when we look at the dye, it appears 
red. When we want to make a blue dye we use 
something that will absorb all the colors except 
the blue we wish. 

Leaves are green because they absorb every 
color but green, and the green is reflected back. 



A prism will bend a ray of sunlight into the colors 
which ma\e up light 


77 




























7 8 


THIS PHYSICAL WORLD 


What ma\e$ 
a rainbow? 


We never see a rainbow except after a shower 
when the sun comes out just as soon as it stops 
raining, or even before it has stopped raining. 
If we are in a place where we can see the whole 
arc of the rainbow across the sky very plainly, 
we can find most of the six colors, red, orange, 
yellow, green, blue and violet. These colors 
show in the sky because there are still rain drops 
in the air when the sun comes out. As the sun¬ 
beams travel from the sun to the earth, they pass 
through the drops of moisture hanging in the 
air high above the earth. When this happens 
the rays of light separate and we see each color 
by itself, as we do in a prism. A rainbow does 
not last long because the heat of the sun evapo¬ 
rates the moisture from the air. As soon as this 
is gone, the colors in the white light are no 
longer broken up. 

When we blow soap nubbles, we see the same 
colors in the bubbles that we see on wet, oily 
asphalt. This happens because very thin films 
of water or oil can break light up into different 
colors. 



The rainbow is sunlight divided into its colors by 
the water drops in the s\y 


79 


















8o 


THIS PHYSICAL WORLD 


What is 

iridescent glass? 


What ma\es 
the s\y blue? 


Is blac\ 
a color? 


Sometimes we get glasses to drink from 
which shows the same colors we see in the soap 
bubble. This colored glass is called iridescent. 
It has been specially made in such a way that 
light is broken up when passing through it. 
When light rays are split in this way, the dif¬ 
ferent colors are reflected back to our eyes by 
this special glass. 

When there are no clouds in the sky and the 
sun is shining brightly, the sky looks blue. Yet 
we know that the sky is nothing but air far 
above the earth. And air is a gas which is 
invisible and has no color. But as the rays of 
sunlight pass through the air, on the way to 
the earth, the molecules of gas which make up 
the air scatter all the colors which make up 
the sunlight. But the shortest colors are 
scattered most by the gas molecules. Violet 
and blue are the two shortest colors, and that 
is why the sky looks blue to us. 

Black is not a color; it is the absence of all 
light. And when there is no light, there can be 
no color, so black is the absence of color. Trees 


WHY WE SEE 81 

and buildings look black at night. Almost 
everything looks black at night out-of-doors 
where there is no artificial light. There is 
scarcely any light from the sun at night, and 
when there is no moon, we do not get enough 
light from the stars to give things color. White 
houses will stand out more clearly at night than 
anything else. That is because white absorbs 
no color, so that even if there is little light, all 
of it is reflected back to us by the white building 
and it shows up better than the dark trees or 
a dark building. 

When we look down into a very deep hole 
that is not very wide, it seems to be black, 
although the hole is really nothing but air. 
Light cannot penetrate to the bottom of the 
hole, and because there is no light down there, 
it looks black. 

White cloth absorbs no color. It throws back 
to us all the light rays that strike it. When cloth 
absorbs light, instead of reflecting it back to 
us, it changes light into heat. A cloth that will 
absorb only one color and reflect the others, will 


Why are we 
cooler in 
white clothes 
in summer time? 


82 


THIS PHYSICAL WORLD 


How does 
a thermos 
bottle wor\? 


have only a little heat. The more colors it 
absorbs, the more heat the cloth will have. 
White cloth does not make any heat, because it 
takes in none of the colors. That is why clothes 
made of white material are the coolest to wear, 
and the most comfortable for hot weather. 

Black cloth is the warmest, because it absorbs 
all the colors and changes them into heat. 

If we put hot coffee or soup or any hot liquid 
into a thermos bottle and cork it tightly, the 
liquid will remain hot for at least a day. But 
a hot liquid put into an ordinary bottle cools in 
a short time. If we take a thermos bottle apart, 
carefully, we can understand why it is able to 
keep things hot. 

A thermos bottle is made of two layers of 
glass inside the metal covering that we see. A 
vacuum is made between these layers, by draw¬ 
ing out all air and sealing the two pieces of glass 
together at the top of the bottle. The hot liquid 
is poured into the bottle, and the heat cannot 
escape because there is no air to conduct the 
waves of heat. 


WHY WE SEE 


83 


But if the bottle were made with ordinary 
glass the liquid would soon cool anyway. The 
reason is that there is a kind of heat that radiates. 



The thermos bottle has a double wall of silvered 
glass with a vacuum between 


And radiating heat can pass through empty 
space on waves of ether . Scientists do not know 
just what ether is, but they use that name for 
matter which exists where there is nothing else. 
There is nothing, not even air, between the 
layers of glass. Yet the heat could get through. 
It cannot be conducted, because there is nothing 
to conduct it. So it is thought that there is 
some invisible matter that carries heat waves, 
and it is called ether. 

The glass in the thermos bottle, however, has 

























8 4 


THIS PHYSICAL WORLD 


How is the 
earth warmed? 


silver on it. This makes it act like a mirror. 
When the radiant heat strikes the silvered glass, 
it is reflected back into the bottle, just as light 
rays would be. In this way it can not get past 
the glass, and the liquid inside is kept hot. 

If we put something very cold in the bottle it 
will remain cold in the same way. The heat 
outside the bottle cannot get in because it is 
reflected back when it strikes the silvered glass. 

The sun gives us heat as well as light. But 
the sun is about 93,000,000 miles away from 
the earth and there is no ordinary matter in 
this space to conduct heat to us from the sun. 
If heat could only reach us by being conducted 
through the air, it would have no way of getting 
from the sun to the layer of air around the earth. 

So we see there must be some other way for 
heat to travel which will explain how it gets 
through all the empty space between the sun 
and earth. Scientists who have studied this 
problem have decided that there is a kind of 
heat which they call radiant heat. 

In the space outside the blanket of air, there 


WHY WE SEE 


85 


is nothing; no molecules of any kind. Most 
scientists think this space is filled with an in¬ 
visible substance not made of molecules and 
which they call ether. This ether is supposed 
to fill all empty space, even the tiny spaces 
between molecules. 

The molecules of the sun are moving about 
very rapidly because the sun is so hot. This 
causes the ether up there to move about and 
make ripples. These ripples spread through 
space with great rapidity. They travel from sun 
to earth in about eight minutes. Heat radiates 
in this way through empty space. When it 
strikes anything, some of it is absorbed, and 
is changed into the usual kind of heat. 
When this radiant heat strikes the layer of air 
around the earth, some of it is absorbed by this 
air, and the rest of it travels through the air 
to the earth. 

Radiant heat is very much like light. But 
heat that is radiating as ether waves cannot be 
seen. Light radiates through space as shorter 
ether waves, and these waves can be seen. 


86 


THIS PHYSICAL WORLD 


How does a 
fire warm us? 


Why do 

some things feel 
warmer than 
others in the 
same room? 


When we build a fire in the grate, it makes 
the whole room warm. It cannot be conducted 
to us through the air, because the air right 
around the flames which is warmed rises up 
through the chimney, and does not come out 
into the room. So it probably is radiated heat 
from the fire which warms the air all over the 
room. The heat travels from the fire on ether 
waves, because there is ether between the mole¬ 
cules in the air. 

When a fire is burning in a room, the tem¬ 
perature is almost even in every part of the 
room, yet some things we touch seem much 
warmer than others. One thing is not really 
warmer than the other. What happens is that 
if we touch one thing it draws a lot of the heat 
out of our hand because it is a good conductor 
of heat, and that makes our hand feel cool when 
we touch that object. We then touch another 
object which is not a good conductor of heat. 
Most of the heat stays in our hand, and the 
object feels warm to our touch. 

Iron is a good conductor of heat. If we leave 


WHY WE HEAR 


87 


one end of a poker in the fire, the whole poker 
will soon get very hot, because the iron conducts 
the heat from the fire and travels from one end 
of the poker to the other. Silver is a good 
conductor of heat. A spoon left in a bowl of 
hot soup will quickly get hot all over. Rubber 
is not a conductor of heat, and a piece of rubber 
is put into a handle where it is joined to a 
pot. This keeps heat from traveling up through 
the handle, and we can hold it without burning 
our fingers. Asbestos does not conduct heat 
and does not burn. That is why theater cur¬ 
tains are made of asbestos, to prevent fire spread¬ 
ing from the stage to the auditorium. In that 
way, if a fire starts, one part of the theater is pro¬ 
tected, and people can get out without being 
hurt. 


Chapter VI 


What is sound? 


WHAT WE HEAR 

W E HEAR with our ears, but sound is not 
made by our ears. We know this be¬ 
cause if we hold our hands over our ears or 
stuff them with cotton, we do not hear very 
well. The sounds we hear are caused by some¬ 
thing outside our ears; they may come from 
close by and be soft noises, or they may come 
from far away and be loud noises. How do all 
these different sounds get to our ears? 

All sound is caused by motion. The motion 
sets up ripples in the air which spread out in 
ripples as water spreads when we drop a pebble 
in a pool. These air-ripples are called vibra¬ 
tions, and they keep traveling until they reach 
our ear-drums. When they strike these we hear 
the sound. 

When someone sneaks, he forces air through 


88 


WHAT WE HEAR 


89 


his vocal cords. This makes the person s throat 
vibrate. These vibrations cause the air around 
his mouth to vibrate and send out ripples. 
When these vibrations reach our ears, they 
cause our ear-drums to vibrate in the same way, 
and we hear the words the person spoke. 

All sound is vibration. Every different move¬ 
ment causes a different vibration and that is 
why we hear so many different kinds of sound. 
If someone plays the piano, the hammers hit 
the strings in the piano. This causes the air 
around the strings to vibrate. These vibrations 
travel through the wood of the piano (because 
wood is a conductor of sound waves) and 
spread through the air until they reach our ears. 
Our ear-drums vibrate in the same way, and 
then we hear the melody that is being played 
on the piano. 

When we play one note on the piano we do 
the same thing as when we play another. Yet 
the pitch, or tone of each note is different from 
the others. That is because the more slowly the 
body vibrates, the lower the tone of the sound 


9 o 


THIS PHYSICAL WORLD 


Can sound 
travel only 
through the air? 


will be. The strings at one end of the piano are 
long and fairly heavy. As we go up the scale, 
each string is shorter and thinner than the one 
before it. The longer the string that is hit, the 
more slowly it vibrates. And the fewer vibra¬ 
tions per second, the lower the tone. 

Air is not the only conductor of sound. There 
are things through which sound is carried more 
quickly than through air. Water carries sound. 
If we hold our heads under water, while we are 
swimming, we hear the people around us. And 
if we hold our hands above the water and clap 
two stones together, we hear the sound they 
make. But if we hit two stones together under 
the water, and keep our head in the water too, 
we hear the sound even more clearly, because 
sound travels better through water than it does 
through air. 

Wood is a conductor of sound. If someone 
stands six feet away from us and takes a watch 
out of his pocket, we cannot hear it ticking 
because air will not carry that small sound far 
enough to reach our ears. But if a log six feet 


WHAT WE HEAR 


9i 


long is lying near us, and we place a watch 
against one end, and bend down and hold our 
ear against the other end, we hear the watch 
tick quite distinctly. 

The harder a material is, the better it carries 
sound. Steel lets sound travel through it more 
quickly than through air. By listening to a 
steel rail we hear the sound of a train coming 
long before we see it or hear it through the air. 

But some materials do not carry sound at 
all. If the walls of a room contain a substance 
which does not let sound waves through, the 
room is soundproof. Noise from inside the 
room cannot get past the walls to the outside, 
and noise outside the room cannot penetrate 
into it. Or if the walls were double, with a 
vacuum between the layers, there would be 
nothing which could conduct the sound 
through and the room would be soundproof. 

We usually hear echoes more often in the 
country than in the city or inside of buildings. 
When in the country we sometimes notice that 
if we shout aloud, the sound of our voices comes 


What ma\es 
a room 
soundproof? 


What ma\es 
an echo? 


92 


THIS PHYSICAL WORLD 


back to us the moment we cease shouting. We 
call this an echo, and what has happened is that 
the sound we make has bounced back to us. We 
may notice a mountain or a stone cliff nearby. 
But even if it is not in sight, there is always apt 
to be some sort of elevation not far off. And 
when we call out, the vibrations travel through 
the air until they strike this elevation. They 
cannot get through it, and instead of stopping 
there, the waves bounce back, just as a ball 
would. They travel back the way they came 
and finally hit our own ear-drums, and we hear 
the sound of our own voices. If the mountain 
is quite near, the echo will come quickly. It 
will take a longer time for us to hear the echo if 
the mountain is farther away. 

Sometimes we hear an echo in a large theater 
or hall when there are only a few people in 
it. We would not hear this echo in the same 
theater or hall, if all the seats were filled. When 
the room is empty, sound waves travel out, and 
when they hit the walls they bounce back at our 
ears. But when the room is filled, the waves 


WHAT WE HEAR 


93 


strike the people and become smothered in 
folds of clothing. 

If we watch and listen to a thunder storm 
from the first stroke of lightning to the last clap 
of thunder, we notice an odd thing. At the 
beginning, lightning comes several seconds 
before the thunder. Sometimes a whole minute 
may pass between the two. Then, as the storm 
grows worse, the thunder follows the lightning 
with hardly a second between, although the 
lightning always comes first. Then, as the 
storm grows less furious, and seems to pass away 
from our part of the earth, again we hear the 
thunder long after the lightning has flashed. 
We know that the same thing causes both 
lightning and thunder, and that they happen in 
exactly the same place. A charge of electricity 
shooting from one cloud to another makes the 
flash in the sky we call lightning. This light 
sends rays down to us in waves. The vibration 
caused by this electric charge also sends waves 
to us which make the sound we call thunder. 
But waves of light travel very much faster than 


Why do we 
see lightning 
before we 
hear thunder? 


94 


THIS PHYSICAL WORLD 


Why does an 
alarm-cloc\ bell 
stop ringing 
when it is 
muffled? 


waves of sound. When the lightning comes 
from very far away, the light waves beat the 
sound waves in their race to the earth. 

Sometimes, when an alarm-clock rings in the 
morning, we are too sleepy to find the lever 
which stops the bell. So we put the clock under 
the blankets to muffle its noise. Putting some¬ 
thing over the bell does not really stop the 
noise, but we no longer hear the bell’s ringing; 
all we hear is a dull sound. This is because 
sound waves do not travel easily through the 
material of the blanket. 

An alarm-clock rings because a knob inside 
it starts to hammer on the bell. This makes the 
bell vibrate. These vibrations travel through 
the air in waves which hit our ear-drums and 
we hear a ringing sound. When we cover the 
clock with a blanket, most of these waves can 
not travel to us and we do not hear the ringing. 
Some of the vibration does manage to send a 
few ripples through, and that is what makes the 
dull noise that we hear. 

A stethoscope is the instrument a doctor uses 



The light travels so much faster in the air than 
the sound that we see the lightning first 

95 















9 6 


THIS PHYSICAL WORLD 


Why does a 
doctor use a 
stethoscope? 


when he wants to listen to our heart-beats. It 
is made of rubber tubing, and looks somewhat 
like a wishbone, a very large wishbone. Two 
ends of it fit into the doctor’s ears, and the other 
end is pressed against the patient’s chest or back 
near his heart. A doctor can hear the heart 
beating if he puts his ear against the patient’s 



The air in the tubes of the stethoscope carries the 
sound to the doctor s ears 


chest right under his heart. But he cannot hear 
clearly enough to be sure that our heart beats 
exactly as it should because the sound is not 
distinct. That is because the vibrations which 
our heart starts when it beats spread out in 
all directions. But when the doctor holds the 







WHAT WE HEAR 


97 


stethoscope tightly against the chest near the 
heart, the waves that ripple out from the vibra¬ 
tion of the heart travel right through the 
stethoscope until they reach his ears. He will 
usually place the stethoscope against the skin, 
and not over our clothing. He does this because 
much of the vibration would be lost in our 
clothing just as it is in the blanket held over 
the alarm-clock. 

When we think how wonderful it is that we 
can hear any piece of music we want, simply by 
letting a needle travel round and round the 
grooves in a record, it seems like magic. But 
when we learn how these records are made we 
see that it is quite simple. When a record is 
made, a soft wax plate is used, instead of the 
hard rubber one we use to hear the music. This 
soft wax has no grooves in it. A soft needle is 
set at the outer edge of it the way we set the 
needle to hear a record. Suppose the record is 
being made by a man singing a song. 

This singing causes the air around his mouth 
to vibrate in a certain way. Every sound makes 


What ma\es 
the music in 
a phonograph? 


9 8 


THIS PHYSICAL WORLD 


a different vibration. The air waves spread out 
into the horn and make the needle vibrate the 
same way. As the record moves round and 
round, the needle makes a wavy line in the soft 
wax which is just like the vibrations the man 
makes in the air when he sings. When he is 
finished, all those waves are marked on the 
record. From this master record, many other 
records are made, and all of them have the same 
wavy lines in their grooves. We buy one of 
these records. We put it on the phonograph, 
place the needle at the edge, and start the record 
moving around. The wavy lines in the grooves 
make the needle vibrate. This makes the air 
around it vibrate in the same way that it did 
when the man was singing. And so we hear 
the same sounds we should hear if the man were 
singing his song in the same room with us. 


Chapter VII 


ELECTRICITY 

T HE face of a compass is like the face of a 
clock, except that a compass is marked 
North, East, South and West instead of having 
numbers. The needle of the compass always 
points north. We may move the compass 
around so that the place which is marked North 
is really turned to the south or east or west, but 
the needle will remain the way it was, pointing 
toward the north pole of the earth. 

The earth is like a huge magnet, with its 
north and south poles like the two ends of a 
magnet, which are also called poles. The com¬ 
pass needle is a magnet too. And one pole of 
a magnet, will always be drawn toward the 
opposite pole of another magnet. We might 
say that the point of the compass needle is its 
south pole. The south pole of the needle is 

99 


Why does 
a compass 
point north? 


100 


THIS PHYSICAL WORLD 


How is a 
compass needle 
magnetized? 


attracted to the north pole of the earth, and that 
is why the point of the compass needle is always 
turned toward the north. 

When a compass is made, the needle is at¬ 
tached in its center to the center of the face of 
the compass. Thus it is free to swing about; 
and no matter how much we turn the whole 
compass around, this needle remains steady, so 
that its point faces north. The compass is of 
great value in keeping a ship to its proper 
course. 

A compass needle is made of steel. It is 
rubbed on a magnet, always in the same direc¬ 
tion. It must be rubbed from one end to the 
other, always starting from the same end, and 
not going back and forth. Each separate mole¬ 
cule in the needle has poles which are called 
negative and positive poles, which are like the 
north and south poles of the earth. Before the 
needle is rubbed on the magnet, the molecules 
are lying every which way, facing in different 
directions. After the needle has been rubbed 
along the magnet, all molecules face the same 


ELECTRICITY 


IOI 


way, their negative poles facing one way, posi¬ 
tive poles facing in the opposite direction. That 
is what is meant by magnetizing the needle. 
Once a piece of steel, or iron, has been magnet¬ 
ized it will remain in this condition even after 
it has been taken away from the magnet. That 
is why steel is used for compass needles. And 
no matter into how many pieces we break a bar 
of steel, after it is magnetized each piece is a 
magnet, with all its molecules facing in the 
same direction. 

Electricity is made of tiny particles, which are 
the smallest things that man knows anything 
about. Nobody has ever been able to see such 
a particle, but men who make a study of elec¬ 
tricity believe that every molecule contains 
particles of electricity, called electrons. And as 
everything is made up of molecules, that means 
that everything contains electrons. 

When a thing contains an average number of 
electrons, nothing happens. It is only when 
things have too few electrons, or too many elec¬ 
trons that things begin to happen. When a 


Why do we 
get a shoc\ 
if we touch 
someone 
after walking 
on a carpet? 


102 


THIS PHYSICAL WORLD 


thing has too few electrons, we say it is positively 
charged with electricity. When a thing has too 
many, we say it is negatively charged with 
electricity. And things get charged with elec¬ 
tricity, either positive or negative, by friction. 

Ordinarily a person has just the usual amount 
of electrons in him. So has a piece of carpet. 
But as this person walks across the carpet, there 
is friction between his feet and the carpet. This 
makes some of the electrons in the carpet rub 
off and enter the person. Then he has more 
than the usual number of electrons and is nega¬ 
tively charged with electricity. Someone who 
has been standing still has less electricity in him 
than the person who has walked across the 
carpet. So that person is positively charged with 
electricity. When the one who is negatively 
charged touches the one who is positively 
charged, he will get a shock, because the extra 
amount of electricity in him jumps the short 
distance to the person who did not have so great 
an amount of electricity. 

Any thing that is positively charged with 


ELECTRICITY 


103 


electricity will be attracted to another thing that 
is negatively charged with electricity. As we 
draw a comb through our hair, there is friction 
between hair and comb. This friction causes 
some of the electrons from the hair to rub off 
and jump to the comb. This extra electricity 
makes the comb negatively charged. And the 
hair is now positively charged. As we bring the 
comb near the hair, the hair reaches out and 
sticks to the comb for a second or two. We hear 
a crackling sound. That is the extra electricity 
jumping back from the comb to the hair. As 
soon as this happens, there is the same amount 
of electricity in hair and comb, and so the hair 
drops back on our head again. 

We notice this happening more in winter 
than in summer. In summer there is more 
moisture in the air. Moist objects are con¬ 
ductors of electricity; that is, the extra electricity 
in the comb goes into the damp air, and the 
comb does not have an extra charge of electricity 
with which to attract the hair. In winter the 
air is dry, and there is thus no place for the 


Why does our 
hair sometimes 
jump out to 
meet the comb? 


104 


THIS PHYSICAL WORLD 


What is an 
electric current? 


What is a 
conductor of 
electricity? 


electricity to gc except from hair to comb and 
back again. 

The kind of electricity got from rubbing 
things together and caused by friction is called 
static electricity. Static means standing still. 
When the electrons jump from the hair to the 
comb, they stay there. Then they jump back to 
the hair, but they do not keep moving after they 
return to the hair. 

Electricity which flows along steadily is called 
an electric current. This is the kind of elec¬ 
tricity we use to make things move, or to give 
us heat and light. Lamps, telephones, street 
cars, heaters, telegraph instruments, and many 
other things are all run by electric current. 

Electricity flows through some things easily, 
other things will not let electricity flow through 
at all, while still others let only a small amount 
through. Water is a good conductor of elec¬ 
tricity. It is dangerous to touch electric appli¬ 
ances when our feet are wet. If at such a time 
we touch a wire, the electricity goes through our 
body and gets from our feet to the floor by going 


ELECTRICITY 


105 


through the water. If our feet are dry, the 
current cannot jump from our feet to the floor 
because there is too great a space between them 
even though we do not notice it. And as long 
as the electricity cannot go into us at one point 
and out through another, we do not receive a 
shock. 

Copper wire is a good conductor of electricity 
and 15 used for all electric wiring. 

Materials through which electricity cannot 
flow are called non-conductors. Rubber is a 
non-conductor of electricity. 

Most electric current runs through copper 
wires laid under streets and brought into houses 
and other buildings in the cellar. These wires 
are double, one to carry the current into the 
house, the other to carry it back to the place 
where it is generated. These wires are covered, 
usually with rubber. This is done so that the 
electricity cannot escape anywhere and be 
wasted, or hurt anyone. 

All wiring, inside of houses, which attaches 
lamps and other things to wall-outlets is bound 


What is 
insulation? 


THIS PHYSICAL WORLD 


106 


Why can 
birds sit on 
telegraph 
wires without 
being hurt? 


with rubber and silk, because they are non¬ 
conductors. When the electric current is 
covered so that it cannot escape, we say it is 
insulated. It must run through the copper wire 
because it cannot get outside the insulation. 

Wires which run from one pole to another 
along country roads are not insulated. The 
wire is not covered with anything and, if we 
were tall enough to reach up and touch the wire 
with one hand, we would be badly hurt by the 
electricity. Birds are conductors of electricity, 
just as human beings are. Yet they sit on these 
wires all day, and nothing happens to them. 

The first thing to notice is that as the bird sits 
on the wire, no part of him touches anything 
else. If we could balance on a power line wire 
without touching the ground anywhere, we 
would not be hurt either. The electric current 
would run through the wire without going 
through us. 

In order for electricity to run through us, it 
would have to be able to get back to the wire 
again. If we did not touch anything else, the 



If we touch the wire when we are standing on 
anything, we get a severe shoc\ 


107 






What happens 
when a fuse 
blows out? 


108 THIS PHYSICAL WORLD 

electricity couldn’t get back to the wire after it 
went through us. But if we stand on the 
ground, electricity can go through us to the 
earth, and through the earth back to where the 
power line wire started. And as the current 
passes through us, the shock is very painful. 
Electricity always forms a circuit. It travels one 
way and comes back by another to the starting 
place. Sometimes instead of having a double 
wire, there is one wire which is grounded. That 
means it is connected with the earth somewhere, 
and so the earth can be used instead of a second 
wire. As the bird sits with both feet on the 
wire, the electricity does not flow through him 
because then it could not complete the circuit. 

Suddenly all the lights in one room, or in one 
part of a building, will go out at once. Someone 
says that a fuse has blown out. Usually that 
person goes to the cellar to fix something, and 
soon all the lights blaze forth again. In the 
cellar a box is placed quite near the spot where 
all the wires come into the building from out¬ 
side. This box covers the electric wires. There 


ELECTRICITY 


109 


may be several different wires leading to differ¬ 
ent parts of the building. Each of these wires 
has its own fuse inside a box. 

Where the fuse is put, the wire has been 
separated and is held together only by a fuse 
made of metal. The copper wire through which 
the electricity flows does not get hot. But if 
a great deal of extra electric current passes 
through it, it becomes hot, and if enough extra 
current passes through, it starts to burn. This 
starts a fire in the house. 

The metal of which the fuse is made will get 
hot very easily. As soon as a little extra elec¬ 
tricity passes through the fuse, the fuse metal 
becomes hot enough to melt. When the fuse 
melts, the two ends of the electric wire are 
separated. This stops the electric current, be¬ 
cause it cannot jump from the end of one piece 
of wire to the end of the other. When the 
current stops, the lights go out. 

An extra flow of electricity usually comes 
from a defect in the wire or in one of the 
electrical appliances, such as a lamp or toaster 


no 


THIS PHYSICAL WORLD 


What causes 
the light in 
an incandescent 
lamp? 


or curling iron, etc. Defective wiring means 
that the electric current can flow through more 
easily than it usually does. Only a certain 
amount of electricity flows through the copper 
wire when it is in proper condition, because the 
wire is always resisting electricity. Sometimes 
there is a defect in the insulation and the elec¬ 
tricity flows through too quickly because there 
is no resistance. We call this a short circuit, 
because it is easy for the electricity to get from 
one wire to another to complete its circuit. This 
makes the wires very hot and causes fire. 

Fuses are cheap, and when they blow out it 
is easy to put in a new one. If we did not 
have fuses, the damage caused by short circuits 
would be serious. 

If we look at the wire that connects a lamp 
with its outlet in the wall, we see it is a double 
wire covered with silk. Electricity flows into 
the lamp through one wire, and out again 
through the other. But as it flows into the 
lamp, it must go through something else before 
it flows out again. In an electric light bulb 


ELECTRICITY 


hi 


there are fine wires that look like thread. These 
are made of a metal called tungsten. Electricity 
flows through these wires, which are called fila¬ 
ments, before it flows back through the second 
wire. And as the electric current passes through 
these filaments, it makes them glow. 

We get this light because the filaments have 
great resistance to electricity. Resistance to 
electricity is something like friction, and friction 
causes heat. The filaments are made of metal 
that has this great resistance to electricity, and 
they are made more resisting because they are 
so thin that there is not room for much elec¬ 
tricity to get through at a time. The filaments 
resist electricity so hard that there is a great deal 
of work done, which makes the metal white- 
hot. Metal will become white-hot when it is 
heated intensely. And white heat gives off 
light. If you can look at an electric light bulb, 
you see the tiny wires are bright and seem to be 
burning. That is what gives us the light. 
After much use, the filaments wear out because 
of the heat that slowly evaporates them. 


112 


THIS PHYSICAL WORLD 


How do we 
get heat by 
electricity? 


In a lamp, the tiny filaments become white- 
hot and give us plenty of light. An electric 
heater may be connected to the same wall plug 
and give us heat instead of light. It will give 
off a little light, like a red glow, but not enough 
to see by. 

There are wire coils in a heater, but they are 
bigger and thicker than the filaments in the 
light bulb. In these larger wires resistance to 
the electric current is not so great. There is not 
so much resistance, and so the wires do not get 
as hot in the heater as do the filaments in the 
bulb. The wires become red-hot, which is a 
degree of heat not as great as white heat. When 
wires get red-hot, they give off a great deal of 
heat, and not much light. The round metal 
part of the heater is used to reflect these waves 
of heat into the room. 
































































































